1. Field of Invention
The present invention relates to a structure of a semiconductor device. More particularly, the present invention relates to a photodiode structure.
2. Description of Related Art
A photodiode is a light-sensitive semiconductor device having a P-N junction that coverts light into an electrical signal (also known as a photodetecting device). Due to the presence of an electric field at the P-N junction, electrons in the N-doped layer and holes in the P-doped layer cannot normally diffuse across the junction in the absence of light. However, when sufficient light falls on the P-N junction, electronhole pairs are generated by energy from the light. These electrons and holes are able to diffuse towards the junction. Due to the presence of an electric field at the junction, electrons will separate out towards the N-side and holes will separate out towards the P-side of the junction and accumulates there. Therefore, a current is able to flow across the P-N junction. Ideally, a photodiode should remain in open-circuit condition in the dark until light is shone on the junction.
In general, photodiode devices are used as imaging sensors in different types of equipment, for example, PC cameras and digital cameras. One major defect of a conventional photodiode is its relatively large junction leakage current. Junction leakage current often leads to the build-up of a large dark current in products that employ a large number of imaging sensors. Furthermore, the large dark current is capable of producing abnormal bright spots on imaging screen.
FIGS. 1A and 1B are cross-sectional views showing the progression of manufacturing steps in fabricating a conventional photodiode.
First, as shown in FIG. 1A, a patterned silicon nitride layer (Si.sub.3 N.sub.4) 102 is formed over a substrate 100. The substrate 100 can be, for example, a P-type substrate or the P-well of an N-type substrate. The silicon nitride layer 102 is mainly used as a mask in a local oxidation of silicon (LOCOS) operation. Next, the LOCOS operation is carried out in an atmosphere filled with water vapor to form a field oxide (FOX) layer 104 on the substrate 100. In other words, an insulating barrier is formed surrounding a device region. Since water vapor and oxygen cannot easily penetrate a silicon nitride layer, silicon dioxide layer does not form in regions covered by the silicon nitride layer 102. Field oxide only forms in regions not covered by the silicon nitride layer 102. However, water vapor and oxygen still manage to get to the corner region below the silicon nitride layer 102 through horizontal diffusion. Therefore, peaked regions (also known as bird beaks) 106 are formed in the corner regions. In other words, a portion of the silicon layer next to the corner region of the silicon nitride layer 102 will be oxidized horizontally to different degrees.
Next, as shown in FIG. 1B, a wet etching method is used to remove the silicon nitride layer 102. Thereafter, an ion implantation is performed, implanting ions opposite in polarity to the heavily doped P.sup.+ substrate 100. That is, the exposed substrate 100 region is heavily implanted using N-type ions to form a heavily doped N.sup.+ region 110. Subsequently, the heavily doped N.sup.+ region 110 is annealed to drive the implanted N-type ions deeper into the substrate interior. Consequently, a photodiode device is formed at the junction between the heavily doped N.sup.+ region 110 and the substrate 100.
However, in a conventional photodiode device, bird's beak regions exist on the field oxide layer on each side of the device. Because stress in those regions is higher and crystal defects occur there more often than in other areas, a large junction leakage current is generated there. Therefore, imaging equipment that employs a large number of these photodiodes can pick up the leakage current to produce a large dark current that will result in the formation of abnormal bright spots on imaging screen.
In light of the foregoing, there is a need to provide a better photodiode structure.